Fuel cell stack

ABSTRACT

A fuel cell stack which comprises a plurality of unit fuel cells laminated via separators, and is connected to external resistors each allowing a feeble current to flow to each unit fuel cell, whereby such problems as an open-circuit voltage generation and corrosion that may be caused by fuel gas remaining after operation shutdown can be resolved. A switch is preferably attached in series with an external resistor.

FIELD OF THE INVENTION

The present invention relates to a fuel cell stack free from thegeneration of an excessive open circuit voltage at the time oflow-temperature start, and the corrosion of constituent members due toan open circuit voltage caused by a gas remaining after operation stop.

BACKGROUND OF THE INVENTION

A fuel cell stack has a structure comprising pluralities of stacked fuelcell units (cells) 1, each of which comprises a membrane electrodeassembly (electrode structure) 2 constituted by an electrolyte membrane201 and catalytic electrodes 202 formed on both surfaces thereof, and apair of separators 4, 4 disposed on both sides of the membrane electrodeassembly 2 via a gas diffusion layer (not shown) such as a carbon paper,etc. as shown in FIG. 22. One separator 4 is provided with fuel(hydrogen) gas-flowing grooves on a surface opposing the electrodestructure 2, and the other separator 4 is provided with air-flowinggrooves on a surface opposing the electrode structure 2. Each separator4 is also provided on a periphery thereof with a projection terminal 121serving as a terminal for outputting cell voltage, which is connected toa voltage-measuring apparatus attached to the fuel cell stack. Todetermine whether or not each fuel cell unit 1 constituting the fuelcell stack is under a normal operation, the voltage of each fuel cellunit 1 is measured by a voltmeter 5 disposed on a lead wire connected toa pair of separators 4, 4 arranged on both sides of each electrodestructure 2 (see FIG. 23).

In a fuel cell stack of such a structure, a hydrogen gas and an oxygengas in the air are reacted to generate electric power. Because the fuelgas remains in the fuel cell stack at the time of operation stop, powergeneration does not immediately stop but continues in each fuel cellunit while the remaining fuel gas and the air exist, resulting in thegeneration of an open circuit voltage between a pair of separators 4, 4disposed on both sides of each electrode structure 2. Thus, workingaround the fuel cell stack immediately after operation stop might resultin short-circuiting or electric shock.

Also, if the fuel cell stack is left to stand in a state in which about1 V of an open circuit voltage exists per a unit cell, the particle sizeof a catalyst on a surface of the electrolyte membrane 201 wouldincrease, and members constituting the fuel cell stack, for instance,metal or carbon separators would be corroded. For instance, in the caseof a separator made of a metal such as stainless steel, etc., eachseparator may be formed by as thin a pressed plate as about 0.1 mm todecrease the laminate thickness of the overall fuel cell stack. In sucha case, corrosion due to the above open circuit voltage may formpenetrating pores in the separator.

On the other hand, in the case of start at such low temperatures as afreezing point or lower, the open circuit voltage becomes extremely highwhen a gas is introduced. In the case of start at −30° C., for instance,1.35 V of an open circuit voltage may be generated, because theelectrolyte membrane 201 is dry. Once current flows in that state, theelectrolyte membrane 201 becomes a water-containing state, resulting indecrease in the open circuit voltage to nearly 1 V.

As described above, because the generation of an extremely high opencircuit voltage is inevitable, it is necessary for an electric circuitto have high breakdown voltage to resist such open circuit voltage,resulting in increase in the cost of a fuel cell system accordingly.

To solve the above problem, there is a method of purging a fuel gasremaining in the fuel cell stack after the operation stop by an inertgas. Because a nitrogen gas is usually used as an inert gas, a tank foran inert gas is necessary to carry out this method. In automobiles,etc., however, not only a space for a tank for an inert gas is needed,but also there are problems of controlling the amount of the inert gasstored in the tank and its supply, making the overall fuel cell systemcomplicated. Accordingly, purge with an inert gas is available only inan experiment fuel cell stack, and its practical use is difficult.

There is also a method of connecting resistors to terminals on bothsides of the fuel cell stack and causing current to flow therethrough sothat a gas remaining in the fuel cell stack is consumed to lower theopen circuit voltage. In this case, the resistors are series-connectedto pluralities of fuel cell units. However, the amount of the remainingfuel gas is not necessarily the same from one fuel cell unit to another,but often different. Accordingly, when current is caused to flow via theresistors connected to the fuel cell units, a reverse voltage is appliedto fuel cell units, in which a fuel gas remains in small amounts andthus is consumed at higher speeds, resulting in the likelihood of damageto the fuel cell units.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide a fuelcell stack having a structure of effectively lowering an open circuitvoltage without damaging each fuel cell unit, thereby solving theproblems of the open circuit voltage and corrosion generated by a fuelgas remaining after the operation stop.

DISCLOSURE OF THE INVENTION

As a result of intense research in view of the above object, theinventors have found that when a fuel gas remains in a fuel cell stackafter operation stop, decreasing an open circuit voltage generated bythe remaining fuel gas by an external resistor connected to each fuelcell unit can solve the problems of damage and corrosion of the fuelcell unit. The present invention has been completed based on thisfinding.

Thus, the fuel cell stack of the present invention comprises pluralitiesof fuel cell units and separators laminated alternately, an externalresistor being connected to each fuel cell unit so that small currentflows therethrough. With this structure, an open circuit voltagegenerated in each fuel cell unit can be decreased by each externalresistor, to prevent the damage and corrosion of the fuel cell unit.

A switch is preferably series-connected to the external resistor. Withthis structure, electric power loss by the external resistors can beprevented during the operation of the fuel cell stack.

In a preferred embodiment of the present invention, a voltage-measuringapparatus is mounted to the fuel cell stack to check whether or not eachfuel cell unit is normally operated; a terminal projecting from aperiphery of each separator is connected to each voltage-inputtingterminal of the voltage-measuring apparatus; and each external resistordisposed in the voltage-measuring apparatus is connected to eachvoltage-inputting terminal in parallel to each voltmeter. The externalresistors are preferably series-connected to each other.

According to a preferred embodiment of the present invention, theterminal projecting from each separator is connected to eachvoltage-inputting terminal of the voltage-measuring apparatus via eachterminal member; and each terminal member is supported by each partitionof an insulating casing having pluralities of partitions in aninsulating state. With each terminal member inserted into each slit ofthe insulating casing having pluralities of partitions, the positioningof pluralities of terminal members can be secured easily, thereby surelypreventing them from being in contact with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view showing a fuel cell stackaccording to one embodiment of the present invention;

FIG. 2 is a perspective view showing a fuel cell stack according toanother embodiment of the present invention;

FIG. 3 is a cross-sectional view showing an equivalent circuit of thefuel cell stack in FIG. 2;

FIG. 4 is a partial cross-sectional view showing a fuel cell stackaccording to a further embodiment of the present invention;

FIG. 5 is a graph showing the relation between current and average cellvoltage in the fuel cell stack;

FIG. 6 a partial perspective view showing an insulating casing forconnecting terminal members to voltage-measuring terminals of separatorsand voltage-inputting terminals of a voltage-measuring apparatus;

FIG. 7 is a front view showing one example of a terminal member suitablyused in the fuel cell stack of the present invention;

FIG. 8 is a development view showing the terminal member of FIG. 7,which is developed along a folding line;

FIG. 9(a) is a cross-sectional view taken along the line A-A′ in FIG. 7;

FIG. 9(b) is a schematic cross-sectional view showing the terminalmember of FIG. 7 and an eyelet mounted thereto;

FIG. 10 is a cross-sectional view taken along the line B-B′ in FIG. 7;

FIG. 11(a) is a partial enlarged view showing the details of a fuel cellstack comprising an insulating casing for connecting terminal members tovoltage-measuring terminals of separators and voltage-inputtingterminals of a voltage-measuring apparatus;

FIG. 11(b) is a partial enlarged, exploded view showing the details of afuel cell stack comprising an insulating casing for connecting terminalmembers to voltage-measuring terminals of separators andvoltage-inputting terminals of a voltage-measuring apparatus;

FIG. 12 is a side view showing an insulating casing, to which terminalmembers are mounted;

FIG. 13 is a side view showing an upper casing;

FIG. 14 is a plan view showing an upper casing;

FIG. 15 is a rear view showing an upper casing;

FIG. 16 is a side view showing a lower casing;

FIG. 17 is a plan view showing a lower casing;

FIG. 18 is a bottom view showing a lower casing;

FIG. 19 is a rear view showing a lower casing;

FIG. 20 is a plan view showing a casing assembled by screwing an uppercasing to a lower casing;

FIG. 21(a) is a schematic view showing a state where each terminalmember and upper comb teeth of the insulating casing are connected to avoltage-inputting terminal, in the connection method of the terminalmember mounted to the casing to the voltage-inputting terminal and thevoltage-measuring terminal;

FIG. 21(b) is a schematic view showing a state where each terminalmember mounted to the insulating casing is rotated around a fulcrum of ashaft engaging the voltage-inputting terminal, in the connection methodof the terminal member mounted to the casing to the voltage-inputtingterminal and the voltage-measuring terminal;

FIG. 21(c) is a schematic view showing a state where the rotation ofeach terminal member mounted to the insulating casing is completed, sothat the terminal member is connected to the voltage-measuring terminaland lower comb teeth of a separator, in the connection method of theterminal member mounted to the casing to the voltage-inputting terminaland the voltage-measuring terminal;

FIG. 22 is an exploded front view showing the structure of a cellconstituting the fuel cell stack; and

FIG. 23 is a partial cross-sectional view showing a fuel cell stack, towhich the present invention is applicable.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a partial cross-sectional view showing a fuel cell stackaccording to one embodiment of the present invention. Each fuel cellunit 1 comprises an electrode structure 2, gas diffusion layers 3, 3constituted by a carbon paper and disposed on both sides of theelectrode structure 2, and a pair of separators 4, 4 disposed on bothsides of the gas diffusion layers 3, 3. The electrode structure 2 iscomposed of a polyelectrolyte membrane and electrode layers formed onboth surfaces thereof, each electrode layer containing a precious metalsuch as platinum, etc. The fuel cell stack is obtained by stacking acombination of the electrode structure 2 and a pair of gas diffusionlayers 3, 3, and the separator 4 alternately. Both surfaces of eachseparator 4 are provided with gas-flowing grooves. However, when theseparator is provided with grooves for flowing a cooling medium, a pairof separators each provided with gas-flowing grooves on one surface andcooling-medium-flowing grooves on the other surface are preferablycombined with the cooling-medium-flowing grooves inside.

Among a pair of separators 4, 4 sandwiching the electrode structure 2,the separator 4 on the side of a fuel gas (hydrogen gas) constitutes anegative electrode, and the separator 4 on the side of air constitutes apositive electrode. Thus, each fuel cell unit generates an electromotiveforce between adjacent pairs of separators 4, 4. The electromotive forceof the entire fuel cell stack can be obtained by connecting pairs ofseparators 4, 4 in series. To check whether or not each fuel cell unitoperates normally, a voltmeter 5 is disposed between a pair ofseparators 4, 4 sandwiching the electrode structure 2.

The fuel cell stack of the present invention comprises an externalresistor 6 connected between each pair of separators 4, 4 to flow smallcurrent therethrough. The external resistor 6 is connected to thevoltmeter 5 in parallel, and both are housed in a voltage-measuringapparatus 10 mounted to the fuel cell stack. Plural external resistors 6are series-connected to each other.

In the embodiment shown in FIG. 1, the external resistors 6 alwaysconnected to the separators 4 should have sufficiently large resistanceso that they do not affect the output of the fuel cell stack.Specifically, the resistance of the external resistors 6 is preferablyset such that power consumption by the external resistors 6 is 1.5% orless, more preferably 0.5% or less of the output of the fuel cell stack.

FIG. 2 is a perspective view showing a fuel cell stack according toanother embodiment of the present invention. In this embodiment, oneexternal resistor 7 is attached to pluralities of separators 4 on oneside, such that the external resistor 7 extends along all the fuel cellunits 1. FIG. 3 shows an equivalent circuit of the external resistors 6in the fuel cell stack of FIG. 2. Each external resistor 6 withsufficiently large resistance not only prevents adjacent separators 4, 4from short-circuiting, but also consumes the remaining fuel gas toreduce an open circuit voltage when the open circuit voltage existsbecause of the remaining fuel gas. Specifically, the resistance of theexternal resistors 6 is set such that the power consumption of theexternal resistors 6 is 1.5% or less, preferably 0.5% or less, of theoutput of the fuel cell stack.

FIG. 4 is a partial cross-sectional view showing a fuel cell stackaccording to a further embodiment of the present invention. The fuelcell stack in this embodiment is the same as shown in FIG. 1, exceptthat a switch 8 is connected to each external resistor 6. Accordingly,only the function of the switch 8 will be explained here.

In order that the generation of an extremely high open circuit voltageis avoided in the case of low-temperature start, for instance, startfrom such a low temperature as a freezing point or lower, the switch 8is closed before introducing a fuel gas into the fuel cell stack, toconnect the external resistor 6 to each fuel cell unit 1, and the switch8 is quickly opened after the fuel gas is introduced. With the externalresistor 6 connected to each fuel cell unit 1, it is possible to avoidthe open circuit voltage from becoming excessively high at the time oflow-temperature start.

With the switches 8 kept open during the operation of the fuel cellstack, the output of the fuel cell stack is prevented from decreasing bythe external resistors 6. Though a fuel gas remains after the operationstop of the fuel cell stack, the switches 8 are closed at the same timeas stopping a load so that the external resistors 8 are connected to theseparators 4, thereby quickly lowering the voltage of the fuel cellstack. This causes current to flow to consume the remaining fuel gas,thereby quickly reducing the open circuit voltage to zero. The switches8 are kept closed until the next start, so that the external resistors 6are kept in a contact state.

The operation of the switches 8 can be automatically controlled by thelevel of current flowing from the fuel cell units to an externalcircuit. For instance, as shown in FIG. 5, the average cell voltage ofthe fuel cell unit tends to increase as the current density decreases,but the corrosion of separators, etc. occurs when the average cellvoltage exceeds a predetermined level Vc. Accordingly, the switches 8are closed when the open circuit voltage becomes Vc or more, and openedwhen the open circuit voltage becomes less than Vc. The resistance ofthe external resistor 6 can be determined from current Ic at voltage Vcas Vc/Ic (Ω).

FIG. 6 shows the overall structure of one preferred example of the fuelcell stack of the present invention comprising the external resistors.This fuel cell stack comprises voltage-measuring terminals (projectionterminals) 121 of separators (not shown) connected to voltage-inputtingterminals 123 of a voltage-measuring apparatus 10 via terminal members101 supported by a casing (only a lower casing 132 is shown). A largenumber of separators have voltage-measuring projection terminals 121 atupper ends of the fuel cell stack on both lateral sides. A large numberof projection terminals 121 on each side are divided to pluralities ofgroups, and the terminal members 101 connected to the projectionterminals 121 in each group are received in one insulating casing 130.

FIGS. 7 to 9 show one example of the terminal member 101 used in thefuel cell stack of the present invention. As shown in FIG. 8, theterminal member 101 is formed by a pair of thin metal plate pieces 101a, 101 a of the same shape connected at one end, which is folded along acenter 101 b. Each thin metal plate piece 101 a, 101 a has a shapehaving portions corresponding to a tip end portion 111, an elasticsupport portion 112 and a fulcrum portion 113. When the thin metal platepieces 101 a, 101 a are folded, the terminal member 101 has asubstantially U-shaped cross section as shown in FIG. 10. Thevoltage-measuring terminal 121 is inserted into a gap in the tip endportion 111, and the voltage-inputting terminal 123 is inserted into agap in the fulcrum portion 113.

In this embodiment, the elastic support portion 112 is constituted by apair of outward curved narrow-width strip portions 112 a, 112 a. Becauseeach strip portion 112 a, 112 a is narrow in width and curved, theterminal member 1 is easily deformed, so that the terminal members 1 canfollow the displacement of the separators not only in a stack directionbut also in two directions in perpendicular to the stack direction, whenconnected to a large number of the stacked separators.

As shown in FIGS. 8 and 9, the fulcrum portion 113 has an opening 115 ata fulcrum position. This opening 115 is aligned with the opening of thevoltage-inputting terminal 123, and rotatably and firmly connectedthereto via an eyelet 118. As shown in FIG. 9(b), the eyelet 118comprises a tube portion 118 a, which is inserted into the opening 115of the fulcrum portion 13, and a flange portion 118 b for fixing thetube portion 118 a. After inserting the tube portion 118 a of the eyelet118 into the opening 115, the tip end portion of the tube portion 118 ais expanded by pressure by a tool, whereby the eyelet 118 is rotatablyand firmly fixed to the opening 115. Because the opening of the eyelet118 functions as a fulcrum when rotated, the terminal member 101 can beprecisely positioned relative to the voltage-measuring terminals 121 andthe voltage-inputting terminals 123.

FIG. 11 shows in detail the relation between the terminal member 101connected to the voltage-measuring terminal 121 and thevoltage-inputting terminal 123, and the insulating casing 130. Theinsulating casing 130 is constituted by an upper casing 131 and a lowercasing 132 both made of plastics, the upper casing 131 supporting thefulcrum portion 113 of the terminal member 101 connected to thevoltage-inputting terminal 123, and the lower casing 132 supporting thetip end portion 111 of the terminal member 101 connected to thevoltage-measuring terminal 121.

FIG. 12 is a side view of the insulating casing 130, and FIG. 13 is aside view of an upper casing 131. FIG. 14 is a plan view of the uppercasing 131, and FIG. 15 is a rear view of the upper casing 131. As isclear from FIGS. 13 to 15, the upper casing 131 comprises an integralbody portion 141, comb teeth 142 having pluralities of narrow-widthslits 143 integrally formed in a forward portion of the body portion 141for preventing the adjacent terminal members 101 from being in contactwith each other, and a ridge portion 144 integrally formed on a rearsurface of the body portion 141. The pitch of the slits 143 is the sameas the pitch of the terminals 121, 123 to be connected in a stackdirection. The ridge portion 144 functions as a handle for rotating thecasing 130. The comb teeth 142 are provided with through-holes 146 in alongitudinal direction. The body portion 141 is provided with a threadedhole 148 opening on a bottom surface.

FIG. 16 is a side view of the lower casing 132, FIG. 17 is its planview, FIG. 18 is its bottom view, and FIG. 19 is its rear view. Thelower casing 132 comprises an integral body portion 151, comb teeth 152having pluralities of narrow-width slits 153 integrally formed in afront portion of the body portion 151 for preventing the adjacentterminal members 1 from being in contact with each other. The pitch ofthe slits 153 is the same as the pitch of the slit 143. The body portion151 is provided with a hole 156 having a unidirectionally extendedcircular cross section at a position corresponding to the threaded hole148 of the upper casing 131, and the opening of the unidirectionallyextended circular hole 156 on a bottom surface is provided with a recess158 for receiving a screw head. To align each slit 143, 153 of the combteeth 142, 152 precisely, a hole 156 having a unidirectionally extendedcircular cross section makes the position of the lower casing 132relative to the upper casing 131 in a stack direction of the fuel cellstack adjustable.

As shown in FIG. 12, when the upper casing 131 is fixed to the lowercasing 132 by a screw 159, both comb teeth 142, 152 are positioned onthe same side, with the slits 143, 153 aligned. What is viewed fromabove is as shown in the plan view of FIG. 20.

FIGS. 21 shows a method for connecting a large number of terminalmembers 101 to voltage-measuring terminals 121 and voltage-inputtingterminals 123 at a time using an insulating casing 130. First, as shownin FIG. 21(a), the comb teeth 143 of the upper casing 131 engage a rowof voltage-inputting terminals 123 in a state where each terminal member101 is inserted into a slit of the insulating casing 130, and eachvoltage-inputting terminal 123 is inserted into a slit having a U-shapedcross section of the fulcrum portion 113 of each terminal member 1. Withthe opening 115 of the fulcrum portion 113, the opening 125 of thevoltage-inputting terminal 123, and the opening 146 of the upper casing131 precisely aligned, the casing 130 supporting the terminal member 101is rotatable around the openings 115, 146 as a fulcrum.

Next, as shown in FIG. 21(b), the casing 130 holding the terminalmembers 101 is rotated to engage the comb teeth 152 of the lower casing132 to the voltage-measuring terminals 121 of the separators, therebyinserting each voltage-measuring terminal 121 into a slit with aU-shaped cross section of the tip end portion 111 of each terminalmember 101. FIG. 21(c) shows a state where each voltage-measuringterminal 121 of each separator is completely inserted into a slit with aU-shaped cross section of the tip end portion 111 of each terminalmember 101.

As is clear from FIG. 11, the tip end portion 111 of each terminalmember 101 sandwiches the voltage-measuring terminal 121 of theseparator, and the fulcrum portion 113 sandwiches the voltage-inputtingterminal 123 of the voltage-measuring apparatus 10, whereby eachvoltage-measuring terminal 121 is connected to each voltage-inputtingterminal 123. In a state where the voltage-measuring terminals 121 isconnected to the voltage-inputting terminals 123 via the terminal member101, each comb piece of both comb teeth 142, 152 of the casing 130serves as a separator for insulating the adjacent terminal members 101.

APPLICABILITY IN INDUSTRY

In the fuel cell stack of the present invention, because ahigh-resistance external resistor is connected to each fuel cell unit,reverse voltage due to the variation of the amount of a gas remaining ineach fuel cell unit can be prevented. Particularly in the case oflow-temperature start, the open circuit voltage, which would reach 1.35V per a fuel cell unit, can be decreased to about 1 V, thereby making itpossible to provide an electric circuit with low breakdown voltage.Also, because the open circuit voltage can be further decreased byalways connecting the external resistors, the electric circuit may havea further lowered breakdown voltage. In addition, it is possible toprevent the parts constituting the fuel cell unit from being exposed tohigh voltage and deteriorated.

A switch connected to each external resistor, which can be opened andclosed at the time of start and stop of operation, makes it possible tosuppress the excess consumption a fuel gas during the operation(application of load), thereby improving fuel efficiency.

1. A fuel cell stack comprising a plurality of fuel cell units and aplurality of separators alternately coupled together, and an externalresistor connected to each fuel cell unit so that current flowstherethrough.
 2. The fuel cell stack according to claim 1, furthercomprising a switch series-connected to said external resistor.
 3. Thefuel cell stack according to claim 1, further comprising a projectionterminal projecting from a periphery of each separator, wherein eachsaid projection terminal is connected to each voltage-inputting terminalof a voltage-measuring apparatus mounted to said fuel cell stack andwherein each external resistor disposed in said voltage-measuringapparatus is connected to each voltage-inputting terminal in parallel toeach voltage-measuring apparatus.
 4. The fuel cell stack according toclaim 3, wherein said external resistors are series-connected to eachother.
 5. The fuel cell stack according to claim 1, further comprising aprojection terminal projecting from each separator, wherein saidprojection terminal of each separator is connected to eachvoltage-inputting terminal of said voltage-measuring apparatus via eachterminal member and wherein each terminal member is supported in aninsulating state by each partition of an insulating casing havingpluralities of partitions.